1. Field of the Invention
The present invention relates, in general, to non-rotating electrodeless high-intensity discharge lamp systems using circularly polarized microwaves and, more particularly, to a non-rotating electrodeless high-intensity discharge lamp system using circularly polarized microwaves, which comprises a waveguide array to propagate microwaves to a discharge lamp therethrough, with an elliptical waveguide arranged in the waveguide array such that the major axis of the elliptical waveguide is rotated to a predetermined angle relative to the horizontal surface of an input waveguide, thus converting linearly polarized microwaves into circularly polarized microwaves due to the rotated angle of the elliptical waveguide relative to the horizontal surface of the input waveguide, and thereby allowing the circularly polarized microwaves to reach the discharge lamp.
2. Description of the Related Art
Generally, an electrodeless high-intensity discharge lamp system excites a circular cavity to the TE11 mode, which is the dominant mode in the circular cavity. Therefore, the microwaves that are transmitted from a rectangular waveguide to a circular cavity that contains a lamp are almost linearly polarized. When the fill in the lamp is discharged by linearly-polarized microwaves, the luminous plasma is formed in the shape of ellipsoid prolate in the direction of the TE11 mode fields. Accordingly, even when the plasma completely fills the entire space inside the discharge lamp, the parts of the lamp that are in contact with the polar zones of the prolate ellipsoidal plasma becomes overheated in the case of an electrodeless high-intensity discharge lamp. Thus, the overheated parts of the lamp are easily punctured or damaged.
In an effort to overcome the above-mentioned problem experienced in the prior art electrodeless high-intensity discharge lamp system, the lamp is rotated using a driving motor. However, the microwave discharge lamp system having such a driving motor requires a complex structure to connect the lamp to the driving motor, thus having a large size and thereby adding expense to the system and reducing reliability. Furthermore, the driving motor will increase the system maintenance frequency due to its shortened lifespan. In order to circumvent the problem of the discharge lamp system having a driving motor, several techniques were proposed to rotate the microwave fields themselves by converting the linearly polarized microwaves into circularly polarized microwaves, as disclosed in U.S. Pat. No. 5,367,226.
In the related art, several methods to circularly polarize the microwaves have been known to those skilled in the art. In the first method as disclosed in U.S. Pat. No. 5,227,698, the waveguide through which the microwaves are propagated to a discharge lamp is divided at a portion thereof into two branches so as to cause a differential phase shift of 90° between two electromagnetic field components in the two branches, and to produce circularly polarized microwaves by combining the two electromagnetic field components with each other. In the second method as disclosed in U.S. Pat. No. 6,476,557, a dielectric material is inserted in a microwave cavity in which a discharge lamp is disposed, so that the dielectric material induces a different phase velocity for the two modes of the coupled microwaves in the cavity. The two orthogonal modes are propagated at different phase velocities and, when combined at the cavity, produce circularly polarized electromagnetic fields in the microwave cavity. In another embodiment of the prior art as disclosed in U.S. Pat. No. 6,476,557, circular polarization is provided from a microwave circuit inserted between a source of microwave power and a cylindrical cavity containing an electrodeless lamp.
However, since the first of the above-mentioned techniques force the electromagnetic fields of the microwaves while decomposing the electromagnetic fields into two orthogonal components, the techniques are problematic as follows. That is, the first technique in which the waveguide is divided into the two parallel branches with different lengths to cause the differential phase shift of 90° between the two orthogonal components of the electromagnetic fields in the two branches, is problematic in that the technique undesirably increases complexity of the structure of the discharge lamp system, complicating the production process of the lamp system and adding expense. Also, it is not easy to stabilize the microwave mode in such devices owing to the interaction between waves that are reflected at the multiple ports. In the second technique, the dielectric material is disposed in the microwave cavity to induce different phase velocity for the two modes of the microwave fields, thereby producing circularly polarized electromagnetic fields in the microwave cavity. The second technique is problematic in that the circular cavity with dielectric material does not set up circularly polarized fields because the waves that is circularly polarized in the initial propagation is reflected back by the end plate of the cavity and it changes the sense of rotation. When such waves are reflected by the first plate which has a coupling aperture, they will have circular polarization in the opposite sense compared to the initial waves, thus restoring the linear polarization. In addition, the use of additional material will add expense and increase the structure of the system.